1. Field of the Invention
The present invention relates to a radio communication system and, more particularly, to a radio communication system which switches transmission modes in accordance with the propagation path quality.
2. Description of the Related Art
As a method of implementing high-speed, high-quality data transmission in a radio communication system, there is a method of switching transmission modes in accordance with the propagation path quality. Although the transmission modes to be switched change in accordance with the propagation path quality, parameters which change the contents are a modulation technique and encoding ratio.
The transmitting side selects, in accordance with the propagation path quality, an error correcting code encoding ratio k/n which is obtained by adding (n−k) redundancy bits to k information bits, and a modulation mode such as QPSK, 16QAM, or 64QAM capable of transmitting 2, 4, and 6 bits, respectively, for each modulation.
As the encoding ratio and the number of modulation bits increase, the maximum data transmission rate increases, but the propagation path quality (indicated by the signal-to-noise ratio S/N or signal power-to-interference ratio SIR) which satisfies the target communication quality (indicated by the block error rate, bit error rate, throughput, or the like) also increases.
In a radio communication system, the propagation path quality fluctuates owing to, e.g., the presence/absence of visibility between radio communication apparatuses, and interference from another radio communication apparatus. Therefore, the system throughput can be maximized if transmission is performed in accordance with the propagation path quality by an optimum one, by which the data transmission rate is maximum, of transmission modes (to be abbreviated as modulation/encoding modes hereinafter) using modulation techniques and encoding ratios which can satisfy the target communication quality.
As a method of realizing the switching between modulation/encoding modes described above, as shown in FIG. 1, there is a method which predetermines the range of the the propagation path quality (dB) as a fixed threshold value, and determines a modulation/encoding mode in accordance with the propagation path quality obtained from a pilot signal which is known to the transmitting and receiving sides. In the example shown in FIG. 7, transmission is performed by the 64QAM modulation technique and encoding ratio R=3/4 when the propagation path quality is 12 dB or more, by the 16QAM modulation technique and encoding ratio R=1/2 when the propagation path quality is 5 dB (inclusive) to 12 dB (exclusive), and by the QPSK modulation technique and encoding ratio R=1/3 when the propagation path quality is less than 5 dB.
The propagation path quality is estimated by the receiving side and notified to the transmitting side, and the transmitting side compares this propagation path quality notified from the receiving side with the threshold value described above, and selects a modulation/encoding mode corresponding to the propagation path quality. However, an optimum modulation/encoding mode changes in accordance with the propagation path environment even for the same propagation path quality. Examples of determining factors of this propagation path environment are the multipath environment (the path count and the delay dispersion) and the maximum Doppler frequency (the moving velocity).
As described above, an optimum modulation/encoding mode changes in accordance with the propagation path environment even for the same propagation path quality. Conversely speaking, if the propagation path environment changes, the threshold value of the propagation path quality for selecting an optimum modulation/encoding mode changes. The larger the change in propagation path environment, the larger the change in threshold value of the propagation path quality for selecting an optimum modulation/encoding mode. Therefore, it is difficult to optimize the threshold value in the method which selects a modulation/encoding mode by comparing the propagation path quality with the fixed threshold value.
As a method of solving the above problem, there is a method disclosed in patent reference 1 (Japanese Patent Laid-Open No. 2003-37554) in which threshold values are variably controlled on the basis of the presence/absence of a reception error for each information block. In this method, a modulation/encoding mode to be selected is determined by comparing the reception quality of a pilot signal with a plurality of threshold values stored in a threshold value table, and the contents of the determination are output as a switching designation. As shown in FIG. 2, a plurality of threshold values are variably controlled on the basis of the presence/absence of a reception error notification from the receiving side. If the reception of an information block is successful, the upper limit of the threshold value range of the propagation path quality for the modulation/encoding mode currently being used is decreased by a predetermined value Pdown dB. If the reception of an information block has failed a predetermined number of times, the lower limit of the threshold value range described above is increased by a predetermined value Pup dB. In this manner, there is provided a radio communication system in which an optimum threshold value for use in the selection of a modulation/encoding mode is set in accordance with the propagation path quality.
Of the above-mentioned conventional techniques, it is difficult to optimize the threshold value if a modulation/encoding mode is determined by comparing the propagation path quality with the fixed threshold value.
The method disclosed in patent reference 1 can determine an optimum threshold value in accordance with changes in propagation path environment. However, it takes a long time to change the threshold value of the propagation path quality to an optimum value, and the larger the change width to the optimum threshold value, the longer the time necessary for converging to the optimum threshold value.